1. Field of the Invention
The present invention relates to a flexible boot for universal joints such as a constant velocity joint used with a drive shaft of an automobile and the like.
2. Description of the Prior Art
A conventional constant velocity joint includes, as shown in FIG. 11, an inner ring 10 fixed to one end of a first shaft 6, an outer ring 5 formed on one end of a second shaft 9, and a flexible boot 1 having a bellows portion 4 and fixed at its both ends to the outer ring 5 and the first shaft, respectively. The flexible boot 1 serves to sealingly hold therein grease used as lubricant for the constant velocity joint and to prevent foreign matter such as dust and water from entering into the joint. The bellows portion 4 of the flexible boot 1 can follow the movement of the joint by expanding and contracting peaks and valleys of the bellows portion 4 even when the first shaft is inclined at a certain operating angle with respect to the second shaft. To this end, the peaks and valleys of the bellows portion are so configurated that they have smoothly curved bent portions for dispersing the stress applied thereon, and the bellows portion 4 is made of material having high elasticity and good resistance to fatigue due to the expansion and contraction thereof, such as rubber and resin.
In the above-mentioned conventional constant velocity joint, however, there was a problem that the valleys 4a of the bellows portion 4 were damaged in a short time when the joint often operated at the large operating angle between the first and second shafts, even if the bellows portion of the flexible boot was made of the material having the good resistance to fatigue due to the expansion and contraction thereof.
More specifically, as shown in FIG. 11, when the constant velocity joint is operated at the large operating angle (i.e., when the first shaft 6 is largely inclined with respect to the second shaft 9 as shown), the valleys 4a of the bellows portion 4 are compressed between an end surface 5a of the outer ring 5 of the second shaft 9 and an outer surface of the first shaft 6, with the result that some of the valleys 4a cannot maintain its smoothly curved configuration, thus creating acutely folded portions B in the valleys 4a. Due to such folded portion B, the condition which exceeds the elastic limit of the material of the bellows is generated in the folded portion (i.e., the condition which changes molecular construction in the material), as in the case where, when a rubber plate is acutely folded, a folded mark is created in the folded portion. When the folded portions B are suddenly expanded due to the change in the operating angle (i.e., the change in the inclination of the first shaft with respect to the second shaft), the molecular construction in the material of the bellows portion will be destroyed. Thus, by repeated folding and expansion of the bellows portion, the valleys of the bellows portion of the conventional flexible boot will be prematurely destroyed. In summary, in the conventional flexible boot, when the valleys of the bellows portion of the flexible boot are acutely folded, the service life of the flexible boot is remarkably or extremely reduced.